While in Austin, Ellis got experience in Texas government, working as an aide to Lieutenant Governor Bill Hobby and as Law Clerk to Chief Justice John C. Phillips on the Third Court of Appeals.

Ellis also served as legal counsel to Texas Railroad Commissioner Buddy Temple before moving to Washington, DC to become chief of staff for U.S. Representative Mickey Leland.

[11] To combat rising drug crime, Ellis pushed to increase funding for anti-drug efforts in the city,[12] but also called for greater community oversight of the Houston Police Department through a citizen's review board.

[23] In 1997, Ellis authored legislation to create the Texas Capital Access Fund that provided up to $140 million in private lending to small businesses and nonprofit organizations.

[28] As chair of the Finance Committee, Ellis managed, in spite of the tight budget, to fund four priority items: a major Medicaid expansion, state employee pay raises, teacher health insurance, and financial aid for college students.

[33] The Act bears the name of James Byrd, Jr., an African American who was targeted and murdered in one of the most brutal hate crimes of the post-Civil Rights Era.

[35] Over the four years prior to the bill's introduction, the Sudanese government and its allied organizations had killed more than 400,000 people and displaced more than 2.5 million in Sudan's Darfur region.

[36] Ellis helped pass the Free Flow of Information Act in 2009 to protect journalists from being forced to testify or disclose confidential sources.

[44] In 2009, Ellis sponsored and passed legislation to establish the Tim Cole Advisory Panel to identify and study the factors that contribute to wrongful convictions.

The Tim Cole Advisory Panel's work led Ellis in 2011 to author a package of legislation to reform and improve the reliability of the Texas criminal justice system.

Those improvements included eyewitness identification reforms to address the leading cause of proven wrongful convictions, and legislation to ensure that DNA evidence can and will be tested—if available—to prove someone's innocence.

[51] In 2013, Ellis authored and passed the "Michael Morton Act," legislation creating a uniform, statutory open file criminal discovery policy in Texas.

[52] With the bill's passage, Texas law now explicitly states that every prosecutor has a duty to disclose documents or information that could raise questions about a defendant's guilt or lead to a lighter sentence if there is a conviction.

[53] The bill was named after Michael Morton, who was wrongfully convicted of murdering his wife and subsequently spent almost 25 years in prison before being exonerated by DNA evidence.

[56] For his work on criminal justice reforms, Ellis was named "Texan of the Year" by the Dallas Morning News on December 26, 2015, along with Senator John Whitmire and Representative Ruth McClendon.

[57] The Dallas Morning News wrote that "the measures [Ellis] championed this year — and in previous legislative sessions — have targeted every major facet of flawed criminal justice, from prosecutors' reliance on junk science (such as bite-mark evidence) and flawed eyewitness testimony, to holding overzealous prosecutors accountable and improving public-defender funding so indigents can't be railroaded into prison.

The bill also required annual filing of personal financial disclosure statements by municipal candidates and officeholders in cities greater than 100,000 and all members of sports and port and authority boards.

[74] As Commissioner, Ellis has played a critical role on the Court, securing major reforms to the misdemeanor bail system,[75] equity guidelines for flood control projects,[76] higher wages and stronger protections for workers contracting with the county,[77] funding for indigent defense[78] and various forms of direct relief for residents in response to the COVID-19 pandemic.

[79] COVID-19 Response As Harris County dealt with COVID-19 and the economic fallout, Ellis led the charge to provide direct assistance to residents, starting with a $30 million COVID-19 Relief Fund in May 2020.

[84] Ellis also voted for a total of $40 million in funding for emergency rental assistance payments for low-income residents to help prevent an eviction crisis in the county.

[85] Criminal Justice In February 2019, Commissioner Ellis voted in favor of a budget increase to the Harris County Public Defender’s Office to $9 million, in order to hire 61 new employees, the majority of them lawyers to represent people charged with misdemeanor, felony, and juvenile courts who cannot afford legal representation.

[88] In June 2020, in response to the death of native Houstonian George Floyd and subsequent nationwide protests against police brutality against Black Americans, Commissioner Ellis led the effort to pass a package of 11 criminal justice reform measures.

[93] Economic Justice In July 2018, Ellis led the effort in securing a $600,000 investment, using money from his Precinct, to fund a county-wide disparity study on the county’s use of minority- and women-owned business enterprises (MWBE) for contracting purposes.

[94] The disparity study results, published in 2020, showed that minority- and women-owned businesses receive only 9% of Harris County’s contract dollars, despite representing 28% of the available market.

[95][96] In response to the report, the Houston Chronicle Editorial Board published an op-ed titled Disparity for minority businesses in Harris County contracts must end.

[103] In December 2019, Ellis voted to approve County Attorney Vince Ryan’s request to consider legal action against Union Pacific, a multi-billion-dollar transportation company for mismanagement of contamination from a rail yard in northeast Houston that has in recent years moved beneath an estimated 110 properties in the Kashmere Gardens and Fifth Ward, both historically Black neighborhoods.

[117] The park is envisioned as a public destination for community reflection, education, organizing and activism around issues of social, criminal, economic and racial justice.

He was instrumental in creating a memorial in downtown Houston for four lynching victims and supported the creation of a board to advise the county on how to preserve black heritage.

[131] Ellis has paid $213,000 of his campaign funds for legal services to provide advice regarding this issue, including to notable criminal defense lawyer Rusty Hardin.

[128] On February 28, 2020, a local news station published surveillance video it had obtained from the storage facility depicting Ellis providing a tour of the African art to parties unknown.