A former police officer who argued she had a right to use lethal force when she killed an innocent man after mistakenly entering his apartment has been convicted of murder.

Amber Guyger faces a lengthy prison sentence after a jury found her guilty of the murder of Botham Jean in Dallas on 6 September last year – a verdict Jean family attorneys hailed as a significant moment in the battle to hold police accountable.

Guyger is white. Jean was black. Guyger is the first Dallas police department officer to be convicted of murder since the 1970s, the Dallas Morning News reported.

Standing in a packed hallway outside the courtroom in Dallas, attorney Lee Merritt told reporters the ruling was “a huge victory not only for the family of Botham Jean, but as his mother, Allison, told us a moment ago, this is a victory for black people in America”.

He said: “It is a signal that the tide is going to change here, [that] police officers are going to be held accountable for their actions.”

Merritt said the community should not have had to wait “on pins and needles” for the conviction of someone who killed a man who was “completely non-aggressive, sitting at home eating a bowl of ice cream and someone barged into his home and shot him to death.”

He added: “This should have been automatic, anticipated, expected, but it is extremely rare. From this day forward we are pushing so that it’s not rare.”

The jury began deliberating on Monday afternoon and reached a verdict on Tuesday morning, with sentencing to follow. Guyger pleaded not guilty. In Texas, murder usually carries a sentence of five to 99 years in prison but Judge Tammy Kemp had allowed the jury to consider convicting the lesser charge of manslaughter.

Jurors, however, decided that Guyger had committed murder.

Ben Crump, a lawyer for the Jean family, told NBC local news, said: “Thank God. Finally America saw the humanity of an unarmed black man who was killed in an unjustifiable way and they returned a verdict that is befitting the criminal, cowardly act of this woman, killing Botham Jean in his own apartment.”

Crump added in a press conference: “This is a precedent now that will go forth across America for equal justice for everybody.”

He said the incident underscored the need for better police training and that the verdict was for “so many unarmed black and brown human beings all across America” who died in interactions with police.

Facebook Twitter Pinterest Amber Guyger in court last week. Photograph: Tom Fox/AP

Jean, a 26-year-old accountant, had settled down on his couch to watch television and eat some vanilla ice cream when Guyger entered his home. She claimed she mistakenly believed it was hers and thought he was an intruder.

Kemp controversially allowed the jury to consider whether Guyger’s conduct could be justified under Texas’s so-called “castle doctrine”. Expanded in 2007, it is comparable to “stand your ground” laws in other states and allows a civilian to use deadly force if he or she “reasonably believes … [it] is immediately necessary” in certain circumstances, such as during a burglary.

Though Jean was on his own property and Guyger the intruder, seemingly inverting the intent of the law, her attorneys argued she made a “mistake of fact” when she went to the wrong home, making her subsequent conduct reasonable. They said she was tired after a long day and many other residents had found themselves at the wrong unit in the past because signage was unclear and floors looked similar.

“She made a series of horrible mistakes,” Toby Shook, one of her attorneys, said. “The law recognises that mistakes can be made.”

Prosecutors said it was “absurd” to believe the 31-year-old’s “commando-style” behaviour was reasonable, especially given her training as a police officer and status as a more than four-year veteran of the department.

They noted that Guyger failed to retreat and call for back-up, questioned the veracity of her claim to have given Jean verbal commands before firing, and pointed out that after calling 911 she appeared to provide only limited medical assistance as Jean, who was from St Lucia, lay dying from a chest wound.

Rather than feeling tired, prosecutors alleged, she was distracted because she had been “sexting” a colleague. Jean had a bright red mat in front of his door that ought to have been impossible to miss.

Guyger – who was fired by Dallas police – wept while testifying.

“I was scared this person inside my apartment was going to kill me,” she said. “I ask God for forgiveness and I hate myself every single day. I feel like a piece of crap.”

In closing statements, Jason Fine, a prosecutor, called most of her testimony “garbage”. Fine said Jean did not act in a threatening manner, but started to stand up “like a normal reasonable person who has somebody busting into his home, and before he can even get up he is shot dead in his own home.

“Killing this man was unnecessary and unreasonable from start to finish.”

Special security measures were put in place during the trial. Jean’s death sparked protests and demands for justice from activists who cited it as one of a long line of racially charged shootings by a police department that lacks accountability. Though she was off duty, Guyger was still in uniform and used her service weapon when she encountered Jean.

Critics of the department have also claimed Guyger was given preferential treatment. It emerged during the trial that the head of the Dallas Police Association told another officer to shut off an audio-visual recording system inside a patrol car so that he could have a private conversation with Guyger soon after the shooting.

She still faces a civil lawsuit brought by Jean’s family.