Deborah Hopkins' departure comes at a precarious time for the company, which is battling to restructure its way out of a debt mountain.
The company has appointed Frank D'Amelio to replace Hopkins. He was previously group president of Lucent's switching business.
In a statement, the company said Hopkins had decided to leave the company to pursue other opportunities now its turnaround plan is in place and the company has the finance available to pursue that plan.
Lucent secured $4.5bn of funding in February, which helped it avoid junk bond status in the US. "Deb [Hopkins] was an important member of the team and helped lay the foundation for many of the systems and financial improvements we saw last quarter," commented chairman and CEO Henry Schacht.
"She was instrumental in helping design the turnaround plan and putting the financing in place to execute that plan," he added.
Of her replacement, Schacht said: "He is a proven leader and I look forward to working with him as we continue with the turnaround strategy."
At the same time, Lucent has revealed plans to merge its switching and data networking units in a bid to cut costs. Janet Davidson, who was president of data networking, will head the combined division.
Lucent is now organised into four divisions - optical networking equipment, wireless equipment, professional services and the new switching and data networking unit.