Hanen began his legal career as a briefing attorney to Joe R. Greenhill, chief justice of the Supreme Court of Texas, from 1978 to 1979.

[5] During the sentencing of a convicted child smuggler in December 2013, Hanen sharply criticized the United States Department of Homeland Security in an opinion that "veered far from the matter at hand" and "that quickly won rave reviews on right-wing news sites."

[6] Hanen wrote, "DHS should cease telling the citizens of the United States that it is enforcing our border security laws because it is not.

"[7] In February 2015, Hanen granted the State of Texas's motion for a nationwide preliminary injunction barring President Barack Obama from carrying out the Deferred Action for Parents of Americans (DAPA) program.

[11] Hanen also ordered U.S. immigration officials to turn over, within weeks, the names and addresses of 50,000 people who received deferral under the expanded deferred action initiative.

[9] On January 19, 2017, Hanen withdrew the sanctions in the order issued on May 19, 2016, as well as his demand that Immigration and Customs Enforcement (ICE) turn over personal information of 50,000 DACA recipients, stating: "This Court finds upon [DOJ's] newly filed evidence, that the statements in question, though repeated on multiple occasions, were not the product of a bad faith intent to deceive the Court or that they were made with malice.

The case was brought about by Palestinian-American contractor Rasmy Hassouna, who had refused to renew his business's, A&R Engineering and Testing Inc., contract with the city of Houston due to the law.

[16] On September 13, 2023, Hanen again held that DACA was unlawful, and extended the ongoing injunction preventing the government from processing new applications for the program.

[20][21] Hanen dismissed the Republicans' lawsuit, stating they needed to prove the existence of an "evil motive" to have the ballots thrown out and he said they failed to do so.

He ordered Harris County to retain and segregate records of any drive-thru ballots cast on Election Day because of the possibility of future legal challenges.

Richard Mithoff, a Harris County attorney said, "We're very grateful that we saved 127,000 votes from both parties," standing with Hollins and other Texas Democrats outside the federal courthouse in downtown Houston.

More than a million registered voters in the county still had not cast ballots, he said, adding that Republicans had filed almost a dozen lawsuits to constrict the intended expanded ability to exercise the franchise.

"[25] He noted the case was the latest in nearly a dozen lawsuits brought by Texas Republicans to block expanded voter access.

In November, Republican Governor Greg Abbott limited every county to a single drop site for mailed votes.

[19] That forced the county to have to close eleven other sites and caused some voters to need to drive many hours to drop their ballots off.

The effort by Hollins that re-opened drive-through voting sites this year was intended to address safety issues resulting from the pandemic.