"Gareth Thomas is a world class player who has had a fantastic season behind him. I have no doubt he has an enormous contribution to make to the tour," said Lions head coach Sir Clive Woodward.
"With him and Jason Robinson coming out, it is a double boost for us."
The Lions squad will reach its full complement of 45 players on Thursday when Robinson, who had stayed home to spend time with his pregnant wife, arrives in New Zealand.
Robinson is likely to be in the frame for selection against Wellington on June 15, 10 days before the first Test.
While Robinson was given extra time at home for compassionate reasons, the Lions have been locked in talks with Toulouse over Thomas' release.
Woodward had hoped Thomas could fly out with the rest of the squad after Toulouse won the Heineken Cup.
Toulouse head coach Guy Noves was desperate to end Stade Francais' two-year dominance of the French championship, but their title bid was ended by a 23-18 defeat.
With Thomas' club commitments now at an end, he is making preparations to leave for New Zealand on Monday.