William Mountfort (c. 1664 – 10 December 1692), English actor and dramatic writer, was the son of a Staffordshire gentleman.
His first stage appearance was with the Dorset Garden company about 1678, and by 1682 he was taking important parts, usually those of the fine gentleman. Mountfort wrote a number of plays, wholly or in part, and many prologues and epilogues. He married, in 1686, Susanna Percival, the actress.
Owing to jealousy of Mrs. Anne Bracegirdle's supposed interest in Mountfort, Captain Richard Hill, an adventurer, who had annoyed her with persistent attentions, accompanied by Charles Mohun, 4th Baron Mohun ambushed Mountfort in Howard Street, Strand, on 9 December 1692. During the struggle Mountfort was stabbed in the chest by Hill, he died of his wounds the following day. Following the attack Hill fled to France. Lord Mohun was tried by his peers and acquitted by a vote of 69 to 14.
The bell of St Clement's Church is reputed to have cracked when tolled at Mountfort's funeral.
His play 'Zelmane' is credited with the first appearance of the expression, "Be still my beating heart."