Saracens had reached the semi-finals of the Heineken Cup and finished at the top of the regular season table to earn a home draw in the last four. But Hartley inspired his side to a first victory of the season over a top three team to reach their first final.
The underdogs outscored their hosts by three tries to one and Hartley will now clash with six more Lions in the final when Leicester play in their ninth successive Premiership final - and go in search of their first win since 2010.
It means there could be as many as seven Lions on display at Twickenham, while up to a further seven could appear in the RaboDirect PRO12 on the same day at the RDS when Ulster Rugby meet Leinster Rugby.
Leinster also play in the Amlin Challenge Cup final in Dublin on Friday, while Welsh prop Gethin Jenkins is in the Toulon squad involved in the Heineken Cup Final in the Irish capital on 18 May and then the semi-finals of the French Championship the following weekend.