Ms Lamont, current party deputy leader, has the support of 27 MSPs, MPs and MEPs. She also claimed the most support from trade unions, other affiliates and party members.
Her rivals Ken Macintosh MSP and Tom Harris MP are backed by a respective 26 and 13 Labour politicians.
Ms Lamont said: “I am delighted that my vision to change Scottish Labour has won such widespread support, winning the most nominations from every section of the electoral college.
“Scottish Labour has to appeal to every part of Scotland, not just our so-called heartlands, and I am pleased to have the backing of constituency Labour parties in the Borders, Aberdeen, Dundee and the Highlands and Islands.
“We also have to appeal to every section of society and I am delighted with the range of support I have received, from the big trade unions to young Labour and Sera, for Labour’s environmental campaign.”
The results of the leadership ballot, which involves the entire party membership, will be announced on 17 December.
Mr Macintosh said he has “widespread backing” for his own bid.