Prosecutors had launched an investigation in June into LafargeHolcim's operations and its possible "financing of a terrorist enterprise".
The source added Olsen was also placed under judicial supervision.
LafargeHolcim had no comment.
Olsen quit as CEO in April after the company admitted it had paid armed groups to keep a factory operating in war-ravaged Syria.
Under French law, the placing of a suspect under formal investigation means that prosecutors believe they have "serious or consistent evidence" that could result in prosecution.
The decision puts Olsen one step closer to trial though the investigation can still be dropped.
Three other individuals were put under formal investigation last week and two others - one of which being Lafarge's previous CEO Bruno Lafont - were still being questioned on Thursday evening, the source said.
France's Lafarge merged with Switzerland's Holcim in 2015. The new entity employs some 90,000 in around 80 countries.
(Reporting by Emmanuel Jarry; Writing by Matthias Blamont; Editing by Alison Williams and Lisa Shumaker)