Bouchard dropped a 6-3, 4-6, 6-0 decision to the 43rd-ranked Mladenovic in her opening match at the Aegon Birmingham Classic on Wednesday in England.
The loss was the Montreal native's 10th in her last 11 matches that has seen Bouchard fall out of the top 10 world rankings.
Next for the No. 11 Bouchard is a trip to Eastbourne, the final tune-up ahead of Wimbledon, where the 21-year-old made a surprise run to the finals last year, losing in straight sets to Petra Kvitova.
At the French Open last month, Bouchard lost to Mladenovic in the first round.
At the start of this season, Bouchard had emerged as a budding star on the women's tennis circuit, having also reached the semifinals at the French and Australian Opens a year ago.