That now leaves only three former Conservative senators hanging on in the red chamber, despite pressure to resign over a raft of inappropriate financial claims and connected police probes.
Liberal Leader Justin Trudeau had faced criticism from his rivals for saying he would welcome Harb back into the caucus once he settled the issue of his living expenses.
That political problem appears to have been taken out of play, in time for the Liberal get-together in Brudenell, P.E.I., beginning Tuesday where financial transparency will be the main focus.
Harb had been fighting the Senate in court over its assessment that he had improperly claimed to be living primarily in a home outside of the national capital region. That complicated legal battle has now been abandoned.
"I have been contemplating retirement for some time as I personally never considered the Senate to be a lifetime position," Harb said in a statement Monday.
"These past few months have been extremely difficult for me and my family and caused me to evaluate what more I could contribute in the circumstances.
"My dispute with the Senate committee on Internal Economy made working effectively in the Senate unrealistic."
Harb says he sent the Senate a cheque for $189,166.17, bringing his total reimbursement to $231,649.07 or the entirety of his living-related expenses.
Land records show Harb has borrowed $230,000 by using mortgages against four properties in and outside the city to get loans from a numbered company owned by an Ottawa lawyer and businessman.
The Canadian Taxpayers Federation quickly pointed out Monday that Harb will still be able to draw from healthy MP and Senate pensions now that he has retired.
Former Liberal Sen. Raymond Lavigne, convicted of fraud and breach of trust in 2011, retired before the Senate was able to strip him of his seat. As a result, he was still able to collect his pensions totalling an estimated $80,000 annually.
The RCMP is still in the midst of an investigation into Harb's expense claims.
Harb continues to defend himself in the expense battle. His lawyer Paul Champ emphasized that an independent audit released last May did not determine he had violated any rules, but rather that the rules themselves weren't clear.
"The Senate committee treated me very unfairly, and I wanted to make the point that every Canadian, even senators, should be entitled to due process," said Harb.
The 59-year-old politician leaves the Senate with just over 15 years left in a term that would have taken him to age 75, the mandatory retirement age. He was appointed to the upper chamber by Jean Chretien in 2003 after 15 years as an MP.
Former Conservative Sen. Mike Duffy paid back his questionable expenses with the help of a clandestine $90,000 cheque from Prime Minister Stephen Harper's chief of staff Nigel Wright earlier this year, setting off the biggest scandal in the government's seven-year history.
Former caucus colleague Pamela Wallin has paid back $38,369 in improper travel expenses, and promised to pay back an additional $100,000.
That leaves only former Conservative Patrick Brazeau on the hook for $48,745. The Senate has said it would begin garnisheeing his wages to recoup the funds.
Brazeau has also complained of unfair treatment by the Senate and unclear rules, but unlike Harb had not hired lawyers to press the issue.
Liberal Sen. Larry Campbell, who is a member of the internal economy committee which ordered Harb to repay his expenses, called Harb a friend when discussing news of the resignation on Monday.
"I'm sad that he is leaving the Senate but I understand completely the situation he finds himself in. And I think he's done the right thing. I think that he's shown responsibility," he said.
Campbell declined to say whether he thinks Duffy, Wallin and Brazeau should also resign.
"I think that he (Harb) did the honourable thing and I leave it up to others to make their decisions on what they want to do."
Campbell added that he doesn't believe Harb — or any of the others — were treated unfairly by the internal economy committee. The process was "totally fair and totally open," he said.
Earlier Monday, NDP Leader Tom Mulcair vowed to keep the Senate spending scandal on the public's front burner, despite the fact Harper has delayed the return of Parliament until October.
"By the time the House finally reconvenes it will have been five months since he's had to answer a question," Mulcair told a room of NDP staffers.
"Well, Mr. Harper needs to answer for himself and he needs to answer to Canadians. If he thought the questions last spring were tough, he hasn't seen anything yet. We're just getting started."