Born at Kilingi-Nõmme, Siimann studied at the University of Tartu from 1965 to 1971. In 1971, he graduated as a philologist-psychologist. From 1989 to 1992, he was the director of the Estonian Television and from 1992 to 1995, Managing Director of Advertising Television Co. He was a member of the Estonian Parliament from 1995 to 1997, and from 1999 to 2003, elected as a Coalition Party member (but since 2001 served as the chairman of the centre-left/social democratic association "Mõistuse ja Südamega" ("With Reason and Heart")).