He first tried to travel to Rome in 1617 or 1618, but made it only as far as Florence, where, as his biographer Bellori reported, “as a result of some sort of accident, he returned to France.” On his return, he began making paintings for Paris churches and convents. In 1622 made another attempt to go to Rome, but went only as far as Lyon before returning. In the summer of the same year, he received his first important commission: the Order of Jesuits requested a series of six large paintings to honor the canonization of their founder, Saint Francis Xavier. The originality and energy of these paintings (since lost) brought him a series of important commissions.

Giambattista Marino, the court poet to Marie de Medici, employed him to make a series of fifteen drawings, eleven illustrating Ovid’s Metamorphoses and four illustrating battle scenes from Roman history. The “Marino drawings”, now at Windsor Castle, are among the earliest identifiable works of Poussin. Marino’s influence led to a commission for some decoration of Marie de Medici’s residence, the Luxembourg Palace, then a commission from the first Archbishop of Paris, Jean-François de Gondi, for a painting of the death of the Virgin (since lost) for the Archbishop’s family chapel at the Cathedral of Notre-Dame de Paris. Marino took him into his household, and, when he returned to Rome in 1623, invited Poussin to join him. Poussin remained in Paris to finish his earlier commissions, then arrived to Rome in the spring of 1624.

Poussin was thirty when he arrived in Rome in 1624. The new Pope, Urban VIII, elected in 1623, was determined to maintain the position of Rome as the artistic capital of Europe, and artists from around the world gathered there. Poussin could visit the churches and convents to study the works of Raphael and other Renaissance painters, as well as the more recent works of Carracci, Guido Reni and Caravaggio (whose work Poussin detested, saying that Caravaggio was born to destroy painting). He studied the art of painting nudes at the Academy of Domenichino, and frequented the Academy of Saint Luke, which brought together the leading painters in Rome, and whose head in 1624 was another French painter, Simon Vouet, who offered lodging to Poussin.

The early years of Poussin in Rome were difficult. His patron Marino departed Rome for Naples in May 1624, shortly after Poussin arrived, and died there in 1625. His other major sponsor, Cardinal Francesco Barberini, was named a papal legate to Spain and also departed soon afterwards, taking Cassiano dal Pozzo with him. Poussin became ill with syphilis, but refused to go to the hospital, where the care was extremely poor, and he was unable to paint for months. He survived by selling the paintings he had for a few ecus. Thanks to the assistance of a chef, Jacques Dughet, whose family took him in and cared for him, he largely recovered by 1629, and in 1630 he married Anne-Marie Dughet, the daughter of Dughet. His two brothers-in-law were artists, and Gaspard Dughet later took Poussin’s surname.

Poussin became acquainted with other artists in Rome and tended to befriend those with classicizing artistic leanings: the French sculptor François Duquesnoy whom he lodged with in 1626; the French artist Jacques Stella; Claude Lorraine; Domenichino; Andrea Sacchi; and joined an informal academy of artists and patrons opposed to the current Baroque style that formed around Joachim von Sandrart. Rome also offered Poussin a flourishing art market and an introduction to an important number of art patrons. Through Marino, he was introduced to Cardinal Francesco Barberini, the brother of the new Pope, and to Cassiano dal Pozzo, the Cardinal’s secretary and a passionate scholar of ancient Rome and Greece, who both later became his important patrons. The new art collectors demanded a different format of paintings; instead of large altarpieces and decoration for palaces, they wanted smaller-size religious paintings for private devotion or picturesque landscapes, mythological and history paintings.

Cardinal Barberini and Cassiano dal Pozzo returned to Rome in 1626, and by their patronage Poussin received two major commissions. In 1628, with a commission from Cardinal Barberini, he completed his first masterwork, The Death of Germanicus (Minneapolis Institute of Arts), the composition of which was derived from the reliefs of the Meleager sarcophagus. In February 1628, he received an even more prestigious commission to paint an altarpiece, the Martyrdom of St. Erasmus, for the Erasmus Chapel in the basilica of St. Peter’s (now in the Vatican Pinacoteca). The original recipient of the commission, Pietro da Cortona, had produced only preliminary designs for the altarpiece when he was unexpectedly chosen to replace Guido Reni on a related project. Cassiano dal Pozzo successful intervention resulted in the re-assignment of the Erasmus commission to his protege, Poussin. As a condition of the commission, Poussin retained Pietro da Cortona’s original design. With its diagonal composition and high narrative drama, the Martyrdom of St. Erasmus is Poussin’s most overtly “baroque” work.

Allegories of death are common in Poussin’s work. One of the best-known examples is Et in Arcadia ego, a subject he painted in about 1630 and again in the late 1630s. Idealized shepherds examine a tomb inscribed with the title phrase, “Even in Arcadia I exist”, reminding that death was ever-present.

Bouyed by this commercial success, Poussin bought a life interest in a small house on Via Paolina for his wife and himself in 1632 and entered his most productive period.

Classical Greek and Roman mythology, history and literature provided the subjects for many of his paintings, particularly during his early years in Rome. His first successful painting in Rome, The Death of Germanicus, was based upon a story in the Annals of Tacitus. In his early years he devoted a series of paintings, full of color, movement and sensuality, to the Bacchanals, colorful portrayals of ceremonies devoted to the god of wine Bacchus, and celebrating the goddesses Venus and Flore. He also created The Birth of Venus (1635), telling the story of the Roman goddess through an elaborate composition full of dynamic figures for the French patron, Cardinal Richelieu, who had also commissioned the Bacchanals. Many of his mythological paintings featured gardens and floral themes; his first Roman patrons, the Barberini family, had one of largest and most famous gardens in Rome. Another of his early major themes was the Rape of the Sabine Women, recounting how the King of Rome, Romulus, wanting wives for his soldiers, invited the members of the neighboring Sabine tribe for a festival, and then, on his signal, kidnapped all of the women. He painted two versions, one in 1634, now in the Metropolitan Museum, and the other in 1637, now in the Louvre. He also painted two versions illustrating a story of Ovid in the Metamorphoses in which Venus mourning the death of Adonis after a hunting accident, transforms his blood into the color of the anemone flower.

As the work of Poussin became well known in Rome, he received invitations to return to Paris for important royal commissions, proposed by Sublet de Noyers, the Superintendent of buildings for Louis XIII. When Poussin declined, Noyers sent his cousins, Roland Fréart de Chambray and Paul Fréart, to Rome to persuade Poussin to come home, offering him the title of First Painter to the King, plus a substantial residence at the Tuileries Palace. Poussin yielded, and in December 1640 he was back in Paris.

When he returned to Rome in 1642, he found the art world was in transition. Pope Urban VIII died in 1644, and the new Pope, Innocent X, was less interested in art patronage, and preferred Spanish over French culture. Poussin’s great patrons, the Barberinis, departed Rome for France. He still had a few important patrons in Rome, including Cassiano dal Pozzo and the future Cardinal Camillo Massimi, but began to paint more frequently for the patrons he had found in Paris. Cardinal Richelieu died in 1632, and Louis XIII died in 1643, and Poussin’s Paris sponsor, Sublet de Noyer, lost his position, but Richelieu’s successor, Cardinal Mazarin, began to collect Poussin’s works. In October 1643, Poussin sold the furnishings of his house in the Tuileries in Paris, and settled for the rest of his life in Rome.

The most famous of his religious works were the two series called The Seven Sacraments, representing the meaning of the moral laws behind each of the principal ceremonies of the church, illustrated by incidents in the life of Christ. The first series was painted in Rome by his major early patron, Cassiano dal Pozzo, and was finished in 1642. It was viewed by his later patron, Paul Fréart de Chantelou, who asked for a copy. Instead of making copies, Poussin painted an entirely new series of paintings, which was finished by 1647. The new series had less of the freshness and originality of the first series, but was striking for its simplicity and austerity in achieving its effects; the second series illustrated his mastery of the balance of the figures, the variety of expressions, and the juxtaposition of colors.

Another important French patron of Poussin in this period was Paul Fréart de Chantelou, who came to Rome in 1643 and stayed there for several months. He commissioned from Poussin some of his most important works, including the second series of the Seven Sacraments, painted between 1644 and 1648, and his Landscape with Diogenes.{{sfn|Wright|1985|p=211} In 1649 he painted the Vision of St Paul for the comic poet Paul Scarron, and in 1651 the Holy Family for the duc de Créquy. Landscapes had been a secondary feature of his early work; in his later work nature and the landscape was frequently the central element of the painting.

In 1647, André Félibien, the secretary of the French Embassy in Rome, became a friend and painting student of Poussin, and published the first book devoted entirely to his work. His growing number of French patrons included the Abbé Louis Fouquet, brother of Nicolas Fouquet, the celebrated superintendent of finances of the young Louis XIV. In 1655 Fouquet obtained for Poussin official recognition of his earlier title as First Painter of the King, along with payment for his past French commissions. To thank Fouquet, Poussin made designs for the baths Fouquet was constructing at his château at Vaux-le-Vicomte.

He lived an austere and comfortable life, working slowly and apparently without assistants. The painter Charles Le Brun joined him in Rome for three years, and Poussin’s work had a major influence on Le Brun’s style. In 1647, his patrons Chantelou and Pointel requested portraits of Poussin. He responded by making two self-portraits, completed together in 1649.

Between 1650 and 1655, Poussin also painted a series of paintings now often called “townscapes”, where classical architecture replaces trees and mountains in the background. The painting The Death of Saphire uses this setting to illustrate two stories simultaneously; in the foreground, the wife of a wealthy merchant dies after being chastised by St. Peter for not giving more money to the poor; while in the background another man, more generous, gives alms to a beggar.

His wife Anne-Marie died in 1664, and thereafter his own health sank rapidly. On 21 September he dictated his will, and he died in Rome on 19 November 1665 and was buried in the church of San Lorenzo in Lucina.

In his later years, his mythological paintings became more somber, and often introduced the symbols of mortality and death. The last painting he was working on before his death was Apollo in love with Daphne, which he presented to his patron, the future Cardinal Massimi, in 1665. The figures on the left of the canvas, around Apollo, largely represented vitality and life, while those on the right, around Daphne, were symbols of sterility and death. He was unable to complete the painting because of the trembling of his hand, and the figures on the right are unfinished.

Cézanne appreciated Poussin’s version of classicism. “Imagine how Poussin entirely redid nature, that is the classicism that I mean. What I don’t accept is the classicism that limits you. I want that a visit to a master will help me find myself. Every time I leave a Poussin, I know better who I am.” Cézanne was described in 1907 by Maurice Denis as “the Poussin of Impressionism”. Georges Seurat was another Post-Impressionist artist who admired the formal qualities of Poussin’s work.

In the 20th century, some art critics suggested that the analytic Cubist experiments of Pablo Picasso and Georges Braque were also founded upon Poussin’s example. In 1963 Picasso based a series of paintings on Poussin’s The Rape of the Sabine Women. André Derain, Jean Hélion, Balthus, and Jean Hugo were other modern artists who acknowledged the influence of Poussin. Markus Lüpertz made a series of paintings in 1989–90 based on Poussin’s works.