The NDP leader says byelection results are not indicators of how a party will perform during the general election scheduled for next fall.
Nevertheless, the trend is hardly encouraging for New Democrats, who've been eclipsed by the resurgent Liberals among byelection voters seeking an alternative to Stephen Harper's Conservative government.
The NDP's share of the vote collapsed Monday in the suburban Toronto riding of Whitby-Oshawa and dropped slightly in the Alberta riding of Yellowhead.
The Conservatives retained both seats but their vote share dropped by nine and 15 percentage points respectively compared to the 2011 election.
The Liberal share, meanwhile, tripled in Whitby and increased seven-fold in Yellowhead.
Monday's results are in keeping with the trend in nine byelections held since Justin Trudeau took the helm of the Liberals 19 months ago.
Liberals have gained anywhere from seven to 37 points in those contests, including snatching one crucial riding — downtown Toronto's Trinity-Spadina — from the NDP and another, Labrador, from the Conservatives.
The NDP has gained vote share in only one contest, in Toronto Centre, where the Liberals gained even more and held the seat. In all the rest, the NDP vote share has dropped one to 20 points.
The Conservatives have lost vote share in all nine, anywhere from four to 25 points.
"Well, you know, I've been around for a while so I also know that byelections are not always a great indicator of the general (election)," Mulcair said Tuesday.
"We've taken account of the result from yesterday," he added. "We know we've got a lot of work to do but we also know that our numbers have never been better heading into a federal (election)."
The NDP, led by the late Jack Layton, vaulted past the Liberals and into official Opposition status for the first time in 2011. The Liberals were left a third party rump on its apparent death bed.
However, since Trudeau won the Liberal leadership, the Liberals have rebounded into the lead in public opinion polls while the NDP has sunk back to its traditional distant third-place slot.