[2] In 1758, Adam, along with his brother Robert, started his business in London (living in Lower Grosvenor Street), focusing on designing complete schemes for the decoration and furnishing of houses.

He then followed in Robert's footsteps by undertaking the Grand Tour, leaving in May 1760 and arriving back in London in October 1763.

Adam succeeded Robert as Architect of the King's Works in 1768 just before work on the brothers' Adelphi project (1768–1772) almost bankrupted the firm (the business employed other notable architects including Joseph Bonomi (1739–1808) who, from 1768, remained with the Adams until 1781).

From 1771 to 1775, he was engaged with his brother in the design and building of Wedderburn Castle near Duns, Berwickshire.

In 1793, he was part of a group of architects that John Soane was asked to supply plans to for the House of Lords.