The same year he was appointed one of the members of the standing commission of the church, and in 1601 was made a royal chaplain, and in that capacity accompanied the Earl of Mar when he went as ambassador to the English court.
He received a pension from the abbey of Arbroath for "service done to the king, and for his earnest care in discharging his ministerial functions, and in the common affairs of the kirk tending to the establishment of discipline", and in 1607 was made titular bishop of Brechin.
He was a member of the assembly of 1610 which allowed spiritual jurisdiction to the bishops, and was one of the three Scottish prelates who were consecrated at London in October of that year.
He was a favourite of King James, and a willing supporter of his measures for the introduction of episcopacy and the English ceremonies, but he was of a conciliatory temper, and the anti-prelatic party had nothing worse to say of him than that he "loved not to be poor".
It is said by the biographers of Samuel Rutherford that, at his admission to the parish of Anwoth, Kirkcudbrightshire, Lamb connived at his ordination by presbyters only.