Trained from early youth in border warfare, he was in August 1542 put in command of Harbottle Castle with a garrison of a hundred men.
On 4 August 1563 he was appointed a commissioner to treat concerning the delimitation of the borders, and on 10 January 1565 to discuss the position of James Stewart, 1st Earl of Moray and other Scottish exiles in England.
Forster at first described it as an accident, but this did not suit the English government, and, with a view to exacting compensation, Russell's death was represented as the result of a deliberate plot.
Meanwhile, various accusations, promoted perhaps by local feuds, were brought against Forster; he was said to have connived at murder, set thieves at liberty, executed others on insufficient grounds, and had dealings with Northumberland wreckers.
He asked the Mayor of Newcastle, Lionel Maddison to keep sure custody of Jacob Kroger and Guillaume Martin, who had stolen jewels from the Scottish queen Anne of Denmark.
He was the father of Sir Claudius Forster, 1st Baronet, who died without issue; and John, who did not inherit the title but succeeded to his brother's estate.