[6][7] Rosenthal grew up in a Jewish family in Flushing, Queens, New York with his parents, Harold (1913–1983) and Esther (1914–1985), and two younger brothers, Erik and Walter.
[4] His father was a high school math teacher and his mother was a left-wing activist[4] who had been a member of the Communist Party in her youth.
[10] His maternal grandmother, Sonia, had immigrated to New York from Russia after the failed 1905 Russian Revolution and was a supporter of the Bolsheviks.
[10] Rosenthal had poor grades in high school and barely graduated, but after nearly failing in college due to the time he spent attending civil rights and anti-nuclear protests, he began to focus on his studies at Queens College, excelling in math.
[3] Rosenthal supervised the Ph.D. theses of fifteen students[15] and the research work of a number of post-doctoral fellows.
With his newfound interest in the law, Rosenthal began volunteering as a paralegal representing friends and activists who had been arrested and charged with minor criminal offences at protests or for civil disobedience or other activist-related offences, particularly related to civil rights or anti-racist activity.
[4] In the 1980s, Rosenthal worked with Roach representing 21 peace activists who had been charged in relation to protests against Litton Industries and their work on manufacturing components for cruise missiles, with Rosenthal arguing that Litton executives were endangering the safety of Canadians through its products.
Rosenthal was one of four University of Toronto professors who sought an injunction to stop Babb along with a declaration by the court that apartheid was a crime against humanity.
[20] Rosenthal provided legal services for various leftist causes and marginalized clients for free.
Rosenthal represented many activists who faced charges as a result of political protests, including Shawn Brant, John Clarke and the Ontario Coalition Against Poverty, Vicki Monague of Stop Dump Site 41, Dudley Laws and the Black Action Defence Committee, and Jaggi Singh and others arrested at the 2010 G20 Toronto summit protests, and wrote articles about some of those cases.
[21] In 2006, Rosenthal represented Indigenous activists at the Ipperwash Crisis and cross-examined former Premier of Ontario Mike Harris over allegedly saying ""I want the fucking Indians out of the park.