The younger Mr Trump made the case for the Trump Organization that the ex-president was a "genius".
He was the first defence witness in the trial, in which he is a co-defendant along with his father and brother Eric.
A loss in court threatens the Trump real estate empire, which could be banned from doing business in New York.
A judge has already ruled Trump significantly inflated the value of his properties by more than $2bn (Â£1.65bn) in order to secure favourable loans, and the trial focuses on charges of falsification of business records, insurance fraud and conspiracy.
In the courtroom on Monday, Mr Trump Jr continued the charm offensive seen in his first appearance two weeks ago for the prosecution, appearing relaxed, cheerful and confident.
Much of the testimony was largely unrelated to the claims at the heart of the case.
Soon after being sworn in, he drew laughs by saying New York Attorney General Letitia James - the prosecutor bringing the case - might sue him for perjury if he said "it's good to be here".
And, nodding to his past complaints about the courtroom sketch art, he said he "already had a talk" with the artist depicting the day's proceedings and joked that he wanted a version of himself with a strong jawline and broad shoulders to be drawn.
He even made Judge Arthur Engoron laugh when he said: "I'm, like, the nongolfer of the family, which has relegated me to the children's table in perpetuity."
He spoke of going from bartending in Colorado to starting as a project manager at the Trump Organization, and then ultimately becoming an executive.
The company dealt with "world class assets" but was like a "mom and pop" business, he said, adding that it operated as a "meritocracy".
Defence lawyers put on a lengthy presentation to introduce evidence of "the Trump story" and recount the company's history.
Mr Trump Jr expounded on various Trump properties, from 40 Wall Street to Seven Springs, to Trump Tower which was the breakthrough project that launched his father's career and "the first time he changed the skyline".
"It would be one of the first great examples of ultra-luxury real estate emerging in Manhattan - the project by which all future high-end residential condominiums would have been judged," he said.
The Mar-a-Lago resort in West Palm Beach, Florida, was "one of the few sort of American castles", with a "virtually unheard of" location that placed it by the lake and also by the ocean.
"You couldn't build that atrium for $18m today," he said, challenging appraisers' assessments of the property's worth. "You need to understand it and see it to actually fully grasp the spectacular nature of this property."
He described his father, who was not present in court, as someone who was on the leading edge of creating value in properties and could turn "eyesores" into "jewels".
"He see the things that other people don't see," Mr Trump Jr said. "He sees the thing that other people would never envision. He plays the long game."
At the heart of the lawsuit are documents known as statements of financial condition, the balance sheets that the Trump Organization used to demonstrate the value of its properties and Mr Trump's net worth so they could obtain loans and insurance rates.
When Mr Trump Jr was on the stand last week, he said he did not work on the financial statements at the centre of the case.
The trial could see Mr Trump barred, along with his fellow defendants, from doing business in New York, and fined at least $250m (Â£204m) in penalties.
After two damaging weeks of testimony already from the Trump family,legal experts told the BBC the case may be beyond salvaging at this point.
The Trump lawyers have alreadyfloated filing for a mistrial, accusing the judge and his clerk of bias.
On Monday, the judge indicated he was willing to listen to how the defence's case plays out.
As prosecutors tried to object at various points that the presentation was irrelevant, he said: "There's no jury - I don't see any prejudice to this."
Later, after an objection to a document offered by defence lawyers, he asked prosecutors: "Do you want to risk a reversal over this one stupid document?"
The state had focused heavily on financial statements and spreadsheets, but Mr Trump Jr's encore set the tone for a defence case that may last into mid-December.
Returning to the stand in his family's civil fraud trial in New York, Donald Trump Jr described his father as "an artist with real estate".