Danny Kirwan, a guitarist, singer and songwriter for Fleetwood Mac whose work fueled the band’s rise during its early years, died on Friday in London. He was 68.

His former wife, Clare Morris, said he died in his sleep after contracting pneumonia earlier this year and never fully recovering from it. Fleetwood Mac also announced his death in a Facebook post.

Mr. Kirwan was only a teenager when he joined Fleetwood Mac in 1968, but his talent was apparent to the band, which at the time consisted of the guitarists Peter Green and Jeremy Spencer, the bassist John McVie and the drummer Mick Fleetwood.

His work was featured on five albums, beginning in 1969 with the bluesy “Then Play On,” on which he shared writing and lead guitar duties with Mr. Green. He also sang all the songs he wrote with the band, which included half the tracks on the gold-certified 1972 album “Bare Trees.”