The trio is one stroke ahead of four players who share fourth place at 2-under 68.
Scherrer, the 2000 Kemper Insurance Open winner on the PGA TOUR, opened with four birdies and one bogey for his 67. He finished the round with eight consecutive pars.
'I played well today,' he said. 'I had to shake off a little rust, but I hit the ball nicely overall.'
A two-time Nationwide Tour champion, Scherrer was just 70th on the money list last season -- a position he hopes will improve following off-season workouts.
'But it is always different hitting that first shot when you know it counts, as opposed to practicing for the last couple of months when the shots don't count,' Scherrer said.
Haas, a PGA TOUR member in 2001 and '05, also posted four birdies and a bogey. He picked up half of his birdies on Panama Golf Club's only two par fives.
Mulroy, a Sunshine Tour winner in his native South Africa, began his rookie season with an eagle, four birdies, a bogey and one double-bogey. His eagle came at the par-5 fourth, his double-bogey at the par-4 15th.
Bob Burns, Erik Compton, John Riegger and Peter Tomasulo share fourth place at 68 -- one shot ahead of 15 players tied at 69.
The Nationwide Tour began its season in Panama for the fourth straight year. Still, it takes time for players to get accustomed to the high temperatures and heavy winds typical in the Central American country.
'This heat is a shock to the system, especially if you played in the snow like I did last week,' Burns said. 'This course can be very tough to score on, especially when the wind is blowing like it is.'
Play was suspended due to darkness at 6:40 p.m. local time. The only three players remaining on the course -- Rafael Gomez, Gustavo Rojas and Rudolfo Gonzalez, all of Argentina -- will return in the morning to finish their remaining two holes.
The second round will begin on time at 7:20 a.m.