Born in Paris, Allegrain was a pupil of Martin, an ornamental sculptor.
By his first marriage, in 1733, he became brother-in-law to Jean-Baptiste Pigalle.
He was accepted by the Académie Royale in 1748 and received as a full member in 1751. His earlier career was dominated by the relationship with Pigalle, with whom he seems to have collaborated, probably in a subordinate capacity, on commissions including the mausoleum of the Maréchal de Saxe and the monument to Louis XV for the Place Royale, Reims.
One of his most celebrated commissions came in 1753, when he worked for Mme de Pompadour on sculpture for her dairy at the Château of Crécy.
