After Cristian Palacios had given Olmedo a 1-0 lead at half-time in the catch-up match, Emelec salvaged a point in Riobamba when Mena's 81st-minute cross from the left wing looped over the hosts' goalkeeper and inside the back post.
The draw extended Emelec's undefeated run to four games with the Primera A's top team leading by seven points from LDU Loja.
Olmedo have not won in the same length of time and sit eighth in the table with 18 points.
Emelec have 34 points after 15 games with a goal difference of plus 19, putting them well ahead of Loja (27, plus two) and third-placed Independiente del Valle (25, plus four).
With the rest of the Primera A playing their Round 15 matches over the weekend, Emelec and Olmedo had a game in hand leading into Wednesday's fixture.
Olmedo opened the scoring in the 28th minute with Palacios getting on the end of a clever flick, before finishing across Emelec's goalkeeper and into the far corner.
The hosts looked set to hold on for a rare win as Emelec struggled, only for the leaders to eventually equalise through Mena.
The two teams are set to do battle again on Sunday with Olmedo on the road at Emelec in Round 16.
