The former Conservative peer made the announcement within minutes of the news Ms Truss is to become the third female Prime Minister in UK history, following in the footsteps of her political idol and the first woman to occupy No 10, Margaret Thatcher.
Ms Truss emerged as the winner in the contest to replace the scandal-plagued Boris Johnson as Tory leader on Monday, having beaten Rishi Sunak in the poll of party members.
Speaking shortly after the result was announced, Lord Ashcroft, who has also served as Conservative Party treasurer, said: “I have watched the Conservative Party leadership campaign with great interest over the past eight weeks.
“Now that the results are in, I am pleased to announce formally that I will be writing a biography of the new party leader and Britain's new Prime Minister, Liz Truss."
He added: “I know that Harry Cole, the political editor of The Sun, and James Heale, the diary editor of The Spectator, are currently working together on a book about Ms Truss.
“According to my contacts in the publishing industry, their effort – which is apparently being produced with the co-operation of Ms Truss and her wider family – will be published this autumn. My project will be different.
“As with all of my previous political biographies, my book on Ms Truss will be accurate, objective, open-minded, fair, factual and even-handed. It will also be independent.”
Lord Ashcroft said that preliminary research he had done on Ms Truss's life, career and character had proved “fascinating".
He said: “It has always been my intention to incorporate into this book an analysis of Truss's first year in office.
“One of my hopes is that this biography might serve as a valuable guide to the electorate in the months leading up to the next general election, which must be held by 2024.”
The book will include comprehensive polling data carried out by Lord Ashcroft plus the reflections of focus groups concerning voters' appetite for a fifth successive Conservative term in government under Ms Truss.
Lord Ashcroft, who is an international businessman and philanthropist as well as being a pollster and an author, has published biographies of David Cameron, Jacob Rees-Mogg, Rishi Sunak and Sir Keir Starmer in recent years.
He added: “I believe that Liz Truss will prove just as rewarding a subject. The multiple challenges that she faces in every department would test even the most experienced leader.
“Is Ms Truss up to the job? I hope to be able to answer that question when my book on her is published in the latter half of 2023.”