Slogging away in rain and stagnant water festering with fever-carrying mosquitoes, they were typically abandoned by their companies when they fell ill, or even fired so the firms would not have to pay their hospital bills.
Poor nutrition and sanitation exacerbated their health problems.
Conditions were not much better after the United States took over the project in 1904, importing 150,000 workers in a massive push to the finish.
Some 5,600 workers died in the breakneck, decade-long American effort, adding to the 22,000 deaths during the French campaign.
Long hours, demanding work
Conditions have changed for the better in the century since the waterway finally opened, but a job on the Panama Canal still means hard work.
Vinueza looks relaxed enough with her enormous sunglasses, jeans and purple nails, but steering her 26.5-meter (87-foot), 6,250-horsepower tugboat around the canal is a challenge in its own right.
"This is my little house. Sometimes I spend up to 16 hours here. I had to give away my dog Aniara because I didn't have time to take care of her," Vinueza told AFP as she steered her tug toward her latest client, the Bahamas-flagged bulk carrier CS Chara.
Guiding huge container ships into the canal's locks takes nerve and absolute control at the wheel.
It remains a profession dominated by machismo. Vinueza, 31, is one of just three women captains on a staff of about 200.
She was the first woman to land the job after the United States handed the canal over to Panama in 1999.