In the 'BattleTech Universe Guide' (I'm reading from the PDF version), near the end of the section 'The Star League Reborn' page 18 it reads...
At the
Second Star League Conference, Precentor Martial Anastasius Focht of ComStar announced his retirement and named Victor as his successor. The new Precentor Martial used his position to vote Coordinator Theodore Kurita into the post of First Lord.
However, on the same page, immediately after that in the next section 'Power Plays' it says...
Despite their best attempts, the Lords of the Great Houses could not put aside their most basic of instincts. Using his position as First
Lord to validate his actions, Sun-Tzu Liao launched a war of aggression aimed at reclaiming the St. Ives Compact, the nation that his aunt had
split from the Capellan Confederation following the Fourth Succession War.
I'm confused by this, which one was the First Lord of The Star League at the time?
EDIT: Or maybe completely disregard what I said above, as I didn't read it clearly the first time. Directly after that it says...
Before the Second Star League Conference even took place, Sun-Tzu had involved troops from every nation in the Inner Sphere in support of his own goals. When First Lord Kurita ended Star League support for his war, Sun-Tzu drafted two Periphery realms—the Magistracy of Canopus and the Taurian Concordat—into a Trinity Alliance that allowed him to swallow his aunt’s realm whole in June of 3063.
So it looks like to answer my own question, Sun-Tzu Liao was the First Lord, and took his action against The St. Ives Compact, before the Second Star League Conference in which Theodore Kurita was voted in as First Lord.
Sorry about that.