Having attracted a substantial live following, Ellis and Wright asked Anderson, who had become the dominant songwriter, to write a hit single. The result was "Living in the Past", which reached No. 3 in May on the UK singles chart and No. 11 in the US, and resulted in an appearance on Top of the Pops. Although other so-called "serious" groups actively resisted issuing stand-alone singles at the time, Jethro Tull felt a hit single was a positive move for the group, if not their priority.
The next album was Stand Up, recorded during April–May and August 1969. It was released in September, and quickly reached No. 1 in the UK charts, the only album by the group to do so. Anderson had now established himself as the group's leader and songwriter, and wrote all of the material, aside from his jazzy rearrangement of J. S. Bach's "Bourrée in E minor BWV 996 (fifth movement)". The album cover unfolded to a photo insert of the band attached to the covers like a pop-up book.
Immediately after releasing Stand Up, the group set off on their first headlining tour in the US, including an appearance at the Newport Jazz Festival. Barre recalled, "It was really the turning point for Jethro Tull—for everything that we were to become and everything we were to inspire in others." The band was invited to play in the Woodstock Festival, but Anderson declined, being afraid that the band would be permanently typecast as hippies, able to play only one musical style.
On 29 January 1970 the band appeared on BBC's Top of the Pops again, performing "Witch's Promise".