Craig Turnbull, a construction law specialist who has been a partner with the firm since 1997, is replacing Michael Murphy. Meanwhile, Edinburgh-based partner Neil Kelly will become chairman. David MacRobert - a fourth generation descendant of the firm's founder - continues as senior partner.
The firm - which has 41 partners and 240 staff across the two offices - will see Murphy returning to full-time client work after six years as managing partner.
Turnbull said: "This is an exciting time to be taking over the managing partner's role and I relish the opportunity to lead the firm forward from its currently very strong base into a steadily improving business and economic climate.
"I should like also to pay tribute to Michael Murphy for his leadership of the firm.
"These have been difficult times for the legal profession and for the Scottish economy as a whole and I am certain that our firm has emerged stronger than it might have as a direct result of his leadership."
Turnbull added that he would continue to carry out some client work in the areas of construction, commercial dispute resolution and health and safety law during his time in charge.
The most recent set of accounts show that MacRoberts' turnover fell to 19.6 million in the year to 30 April, 2010, from 20.1m in the previous 12 months. Average profit per equity partner - formally "members" of the firm's limited liability partnership - fell to 155,359 from 180,975.
MacRoberts is the second large Scottish law firm to change its boss within a week after Chris Smylie become chief executive at Maclay Murray & Spens.