In 1661 Lamy was appointed professor of classics at the Jesuit Collège de César in Vendome. He held this post for two years before being appointed to a similar post at the Jesuit College of Juilly. While teaching literature, grammar, Latin, Greek, history and geography at Juilly, Lamy was ordained to the priesthood in 1667. During his time at Juilly, Lamy read all of Descartes' major works. Then he taught at Le Mans from 1668 to August 1669. Following this he studied theology at the École de Théologie de Notre-Dame des Ardillires at Saumur and was then named professor of philosophy at the Collège Royal des Catholiques in Saumur on 9 September 1671. He moved to a similar post at the College of Angers, part of the University of Angers, two years later. Kennedy writes :-
The most original rhetorical treatise to appear in France in the seventeenth century was 'De l'Art de parler,' or 'On the Art of Speaking,' published anonymously by Bernard Lamy in 1675. Anonymity was probably sought to protect Lamy and the religious order to which he belonged from charges of the influence of Descartes, whose religious views had come under scrutiny by the Church. In 1676 an English version of Lamy's book, with some departures from the French text, was published in London. The translators, who have not been identified with certainty, attributed the original to "Messieures du Port Royal." Lamy was not a member of the Port Royal group, but he had benefited from some of their thinking about language and logic, and readers easily accepted the new rhetoric as the counterpart of the well-known Port-Royal Logic and grammars, thus greatly increasing its sale. The English edition was repeatedly reprinted in the eighteenth century but without inclusion of revisions that Lamy later made in the French text.Publishing this work anonymously did not save Lamy from trouble, however, since through his teaching at Angers he was known to be someone with a great respect for Descartes' philosophy. In Angers :-
... his teaching was attacked on the ground that it was too exclusively Cartesian, and Rebous the rector obtained in 1675 from the state authorities a decree forbidding him to continue his lectures.In  Beaude gives further reasons for the attacks on Lamy in addition to his high regard for Descartes' views, writing that Lamy was:-
[a]ttacked and denounced for Augustinianism, Cartesianism and antimonarchical opinions.Following the issue of the decree, Lamy was exiled by order of the King early in 1676 and he went to live alone in Saint-Martin de Miséré in Dauphiné. In 1677, Guillaume Quesnel, who was the brother of Pasquier Quesnel, became the Superior of the Seminary at Grenoble. Pasquier Quesnel was a friend of Lamy, and it is thought that he approached his brother to try to ensure that Lamy was able to return to teaching. Certainly, Lamy's exile ended, thanks to the decision of Cardinal Le Camus, and he was able to teach again in the seminary in Grenoble.
We mentioned above Lamy's anonymous publication of De l'Art de parler in 1675. Kennedy  describes the work and also give a delightful quotation:-
Although Lamy admired and quoted Cicero, Quintilian, and Augustine, his handbook is not a traditional work. Aristotle is dismissed with little notice. The influence of Ramism is slight, that of Cartesian method strong, and there are many points of contact with the Port-Royal Logic. Lamy reorganised the structure of rhetoric to begin with an account of language: the organs of voice and speech; the parts of speech; the need to use words in their proper sense. But language, he recognised, is not rich enough to supply terms for all ideas; thus in the second part he proceeds to consideration of tropes and then to figures as expressive of the emotions. The third part of the work discusses sounds, pronunciation, and delivery. The fourth part examines style in a larger sense: imagination, memory, and judgment as the basis of good style; the three levels of style; the lofty, the simple, and the middle; and the differences between styles of an orator or preacher, a historian, and a poet. Only in the final section of the work does he come to any means of persuasion, the invention of proofs, dialectical topics, and the arrangement of a speech into parts. Although speech is the subject of the work, Lamy has much to say about poetry, including versification. In one passage Lamy briefly states what might be called the dogma of neoclassical aesthetics. In the English version the text reads: "A discourse is beautiful when it is composed according to the Rules of Art; it is great when it is more than ordinary perspicuous; when there is not one equivocation; no sentence unintelligible; no expression ambiguous; when it is well-disposed, and the mind of the Reader led directly to the end of the design, without the remora or impediment of impertinent words. Such clearness like a Torch dispels all obscurity and makes every thing visible."Lamy's next major publication was Nouvelles réflexions sur l'art poétique (1678) followed by Traité de Mécanique, de l'équilibre des solides et des liqueurs in 1679 in which the parallelogram of forces law is given. Pierre Varignon discovered the parallelogram of forces law independently, at about the same time, and he saw more consequences of it than did Lamy; see  for more details. Lamy also published Traité de la grandeur en general, qui comprend l'arithmétique, l'algèbre, l'analyse (1680) and Entretiens sur les sciences (1783) in which :-
... he proposes an art of learning and teaching all the secular and religious disciplines. This book, admired by Rousseau, is simultaneously an educational treatise, a discourse on method, and a guide to reading.Lamy then published Les éléments de géometrie, ou de la mesure du corps, tout ce qui comprennent qu' Euclide a enseigné (1685).
In 1686 Lamy obtained permission to live in Paris, spending a while at the seminary of Saint Magloire, but trouble over a theological work had him sent away from Paris in 1689. Around this time he published Apparatus ad Biblia Sacra, in quo Hebraeorum de gente, Legibus (1687) which was translated into both French and English. The English translation was published in 1723 as Introduction to the Holy Scripture in three books. Next came Demonstration de la vérité et de la sainteté de la morale chrétienne (1688), and Harmonia sive Concordia quatuor evangelistarum, in qua vera series actuum et sermonum Domini nostri (1689). In this work, a harmony of the Four Gospels, Lamy made claims which went against the beliefs of the Church. For example, he claimed that John the Baptist was put twice in prison, first by Sanhedrin in Jerusalem then later by Herod in Galilee. He claimed that Christ's crucifixion took place on the Passover, and that three Mary's, namely Mary Magdalen, Mary the sister of Lazarus, and the sinner referred to by Luke are the same person. These views landed Lamy in severe controversy with many within the Church.
He lived in Rouen from 1690, remaining there for the rest of his life. He published many books while in Rouen, mostly religious works, including such as Traité historique de l'ancienne Paque des Juifs (1693), Commentarius in concordiam evangelicam (1699), Dissertation sur Sainte Madeleine (1699), and Défense de l'ancien sentiment de l 'glise latine touchant l'Office de Sainte Magdelaine (1699).
In 1701 he published Traité de perspective où sont contenus les fondements de la peinture which was translated into English as Perspective made easy or the kind of representing all manner of objects (1710). The authors of  write:-
In previous publications [Lamy] had addressed the fields of rhetoric, mechanics, mathematics and geometry, and his discussion of perspective gains force and depth from his interest in the study of optics. Lamy conceives of optics as dependent on the practices alike of the scientist, the mathematician and the painter. His readiness to accord equal significance to the skills of each is one of the distinguishing features of his work. It might seem that his belief that mathematics is the 'foundation of painting' was not one to which all contemporary styles of painting could have been made to conform. Lamy's sophisticated argument, however, is that any representation will fail if its effect is not in some way equivalent to the experience of its object.This intriguing description of Lamy's work prompts us to quote a little from the work (at least from the English translation of 1710, which we've put into modern English):-
To represent upon cloth what is not there as cavities and eminences, where all is flat; and distances when every thing is near; is a performance that merits admiration. It is an effect and, at the same time, a proof of what the eye (to speak philosophically) does not see ... Nothing is more difficult to be expressed than the nature of these images ...
A picture may be considered as an open window or transparent glass, through which the eye that is supposed to be at a certain point, may see the objects represented by the picture. Now by the help of the mathematics, the passage of the rays which render the object visible may be traced in the picture or transparent glass. This passage being marked with suitable colours and, in a word, all their appearances. ... Mathematicians draw only lines, they cannot finish a picture. And on the other hand, painters cannot begin it without a regard to the rules taught by the mathematicians. Every picture is a perspective, so that what is taught in this part of the mathematics is the foundation of painting ...
Article by: J J O'Connor and E F Robertson