[5] An activist during the early 1990s and the feminist movement, Liss-Riordan co-founded the Third Wave Direct Action Coalition with writer Rebecca Walker[6][7] to "mobilize young women.

[20] This line of cases developed a previously unused 1952 Massachusetts law protecting tipped employees and resulted in verdicts against the Hilltop Steak House, the Federalist restaurant, and other settlements including against the Four Seasons Hotel, the Weston Golf Club, Northeastern University, the Palm, Ruth's Chris, and Starbucks.

[28] The cases received extensive coverage from the Boston Globe, including an editorial “Kicked at the Curb”,[29] and earned Liss-Riordan the nickname “Sledgehammer Shannon”.

[32][33] She challenged the cleaning industry for misclassifying mostly immigrant workers as franchisees, winning victories against companies including Coverall and Jani-King.

[35][36] Liss-Riordan continued to challenge them in other states, including California, where she won rulings that Jan-Pro misclassified franchisees and had to reimburse them for expenses.

[40] Some strippers, including Stormy Daniels, a spokesperson for the chain Deja Vu, argued that a change in employment status would hurt dancers,[41] but the Boston Globe praised the litigation in an editorial, writing “Attorney Shannon Liss-Riordan, a labor law specialist, can now add strippers to the list of skycaps, baristas, wait staff, and other workers who sought her counsel and received justice.”[42] In 2010, Liss-Riordan sued Upper Crust, a Boston-area pizza chain, for demanding that its Brazilian workers repay sums that their employer was ordered to pay them following an overtime investigation by the U.S. Department of Labor.

[48] Beginning in 2013, Liss-Riordan filed suits against a number of tech companies in the "gig economy",[49] including Lyft, Uber[50] and Amazon.

[54] Liss-Riordan is best known for leading a class action case on behalf of Massachusetts and California drivers against ride-sharing company Uber, filed in 2013 and known as O'Connor v.

[55][56] The case argued that Uber drivers were misclassified as independent contractors, which she said allowed the company to "save massively by shifting many costs of running a business to the workers, profiting off the backs of their workers”[33] Because of that case and similar ones she litigated against other gig companies, in 2015 the Wall Street Journal called her "one of the most influential—and controversial—figures in Silicon Valley.

[65] In 2018, Liss-Riordan brought an age discrimination class action against IBM, alleging that the company was working to "build a younger workforce" by laying off thousands of older workers.

[67] Through the litigation, Liss-Riordan uncovered documents showing top executives referring to older workers as “dino-babies” and plotting how to make them an “extinct species,” which were reported in The New York Times.

[76][77] On January 25, five days after incumbent Maura Healey announced that she would vacate the seat in order to run for governor, Liss-Riordan launched her campaign for attorney general.

[78] She was also endorsed by over 80 elected officials, including US Senator for Massachusetts Elizabeth Warren and Boston Mayor Michelle Wu.