The 27-year-old Sussex southpaw, who shared 141-run partnership with Jonathan Trott (44, 5x4) for the fourth wicket, hit 20 boundaries in his chanceless 151-run unfinished knock, which came off 291 balls.
The visitors, who are leading the group having bagged full points from their previous match against Central Zone, now have a fare chance of making it to the final if they manage to draw the match against West with first innings lead as the latter have only three points in their kitty.
West Zone made an early breakthrough after new-ball bowler Ashraf Makada trapped Lions' opener Michael Carberry (1) for a LBW decision. However skipper Yardy steadied Lions' ship with two fruitful associations. He put on 50 runs along with Joe Denly (32, 6X4s) and then milked 75 runs for the third wicket stand with English discard Ed Joyce (32, 4X4s).
Yardy, who had a good outing with bat in the previous tie against Central Zone with a classy half-century, batted throughout the day with elan. He was comfortable against pace and spin.
Denly, who survived an easy catch-behind chance in the 13th over, was run out in a bizarre fashion after a straight drive from Yardy dislodged the stumps at non-striker's end deflecting off the bowler Siddarth Trivedi's foot and Denly was found out of crease to leave England-A at 126/3.
Soon after lunch Ed Joyce, who looked set for a big knock, was undone by a yorker length ball from Makada that rapped the Irish born England batsman on the backfoot for a plumb LBW decision. Trott joined Yardy in the middle and took Lions past 250. Trott was dismissed minutes before the close of play for LBW decision off Makada.
Makada was the pick among West bowlers on day one with 3 for 57 in 16 overs.
Brief Scores (D-1):
England Lions: 273/4 (Michael Yardy 151 n o, Trott 44, Ed Joyce 32, Joe Denly 32; Makada 3 for 57) Vs West Zone