Forster was the son of William Forster of Aldermaston House and his wife Elizabeth Tyrell, daughter of Sir John Tyrell of Heron, Essex. He was grandson of Sir Humphrey Forster, 1st Baronet (of Aldermaston). He succeeded to this Baronetcy and the Aldermaston manor and leased rectory on the death of his grandfather in 1663, his father having died in 1661.
He was re-elected Member of Parliament for Berkshire in the first election of 1679 but not in the second. He was elected Member of Parliament for Berkshire again in 1685. and then in 1690 and 1695. He was Sheriff of Berkshire in 1704.
Forster died at the age of about 62, when the Baronetcy became extinct and is buried in the church of Street Aidan with his distant ancestors who received the earlier Forster baronetcy in Bamburgh, Northumberland and who held Bamburgh Castle for two centuries until the English Civil War.
His largest estate left the related Achard-Forster families in whose hands it had been since a 12th-century grant by Henry I of England in 1762, when it passed into a family due to a marriage, the Congreve family of the manor house, Congreve, Penkridge, Staffordshire. The estate had to be sold on a bankruptcy (following fire) and has been more than halved with a rebuilt manor house in Elizabethan style.
Early in the 17th century the family funded a vestry south of the Church of Street Mary the Virgin, Aldermaston and their own memorial vault beneath this. Four polished marble slabs in the floor commemorate:
Humphrey"s daughter, Rebekah, daughter age 12 in 1676.
Ann, daughter of William Forster, daughter in infancy in 1664.
John Forster, daughter 1674
Humphrey"s mother, Lady Anne Forster, daughter 1673. On the south wall of the chancel are two stone slabs and lengthy family tablet from the vault entry, transcribed into the county history of 1923.
Cavalier Parliament. Habeas Corpus Parliament]
In 1677, Forster was elected Member of Parliament for Berkshire in a by-election to the Cavalier Parliament.
