MacLennan originally had planned to pursue a career in print journalism but switched to broadcasting after a college internship at CJOH-TV in Ottawa, Ontario. MacLennan co-anchored Canada AM on CTV from 1985 to 1987.
In 1986, MacLennan was struck by a policeman on a motorcycle in Toronto and developed a blood clot on the brain. After doctors failed in their efforts to dissolve the blood clot with drugs, they operated on her at substantial risk to her. She eventually made a full recovery.
In March 1987, MacLennan left CTV to join WBBM-TV in Chicago as a reporter and late afternoon co-anchor. In April 1989, MacLennan became WBBM-TV's 10 p.m. co-anchor, replacing Walter Jacobson in the anchor chair alongside Bill Kurtis. Jacobson had been the station's 10 p.m. co-anchor since 1973.
In February 2000, MacLennan was demoted as WBBM's 10 p.m. co-anchor as both she and Holt were demoted in favor of a solo anchor experiment involving Carol Marin. After station bosses pulled the plug on Marin as solo anchor in October 2000, WBBM then made MacLennan an interim 10 p.m. co-anchor, alongside David Kerley. However, in December 2000, WBBM hired Tracy Townsend as a 10 p.m. co-anchor alongside Kerley, and MacLennan shifted to being a solo 11 a.m. news anchor (replacing Jay Levine and Mary Ann Childers) and a 4:30 p.m. co-anchor alongside Vince Gerasole. In 2001, MacLennan got a co-anchor for the 11 a.m. newscast, Michael Ayala.
In April 2002, MacLennan again returned to co-anchoring the 10 p.m. news, this time supplanting Townsend alongside the newly hired Antonio Mora.
With a new general manager, Joe Ahern, on board, MacLennan abruptly took a contract buyout from WBBM in February 2003.
As of June 2017, MacLennan became a part-time anchor for WBBM-AM in Chicago.
In August 1992, MacLennan married Chicago attorney David Rammelt. They have three children: Taylor Albert, Carson Charles and Charlotte Grace Nelson Rammelt. The couple divorced in 2014. She lives in Kenilworth, Illinois.
MacLennan is a naturalized U.S. citizen.