SEATTLE — A frugal Seattle attorney who nurtured a secret fortune for more than 40 years before his death in September has bequeathed Seattle Children’s Research Institute the largest charitable gift in the hospital’s 106-year history.

Jack MacDonald, who died in September at the age of 98, left a $187.6 million charitable trust split between Children’s, the University of Washington School of Law and the Salvation Army.

It is the largest philanthropic gift in Washington state this year, and the sixth largest in the country in 2013, Children’s officials said.

MacDonald, who had no children of his own, worked for three decades as an attorney for Veterans Affairs in Seattle. He took his pay, along with money he inherited from his parents, and became a student of the stock market even while clipping coupons and hunting for bargains, said his stepdaughter, Regen Dennis.

Only his family and close friends knew he was amassing a fortune.

In his later years, as a resident of Horizon House retirement community, he purposefully wore sweaters with holes in the elbows because he didn’t want anyone to find out how wealthy he was, Dennis said.

“I thought of him in many ways as a gentle giant,” said Doug Picha, president of the Seattle Children’s Foundation, who was a friend of MacDonald’s for 30 years. “He was tall, very shy, very understated, humble. You would never have known that he had great wealth.”

During his life MacDonald supported hundreds of causes with smaller donations, including $536,000 to Children’s. At the same time, he was working to build a much larger trust to leave the three institutions at the time of his death.

“He was amazing,” Dennis said. “He didn’t trust a lot of other people to do his research; he directed what he wanted bought, and he really knew what he wanted.”

Forty percent of the trust’s yearly income, or about $3.75 million in the first year, will support pediatric research at Seattle Children’s. It is the largest single gift ever made in the nation in support of pediatric research, Children’s officials said.

“The impact of that size of a gift is really quite profound — it’s transformational in many ways,” Picha said.

Some of the money will be used to provide matches to other donations, increasing the power of MacDonald’s donation. That bequest honors his mother, Katherine MacDonald, who was a member of its Connie Beal Howard Guild, which raises money for the hospital.

MacDonald’s pledge to Children’s was first announced in 2011, and at that time was anonymous.

Another 30 percent will go to the Jack MacDonald Endowed Chair, student scholarships and general education needs at the UW law school. It’s the largest gift to the law school in its history, and the largest estate gift to the UW as a whole. MacDonald graduated from the UW Law School in 1940.

The remaining 30 percent will support the Salvation Army Northwest Division, which will receive a portion of the annual interest, approximately $2.8 million in the first year. That gift honors MacDonald’s father, Frederick MacDonald, who wished to help men and women in need.

Salvation Army officials said the gift came as a surprise.

“We didn’t know him, but he definitely knew us,” said Major Douglas Tollerud, divisional commander for the Salvation Army, in a statement.

MacDonald was born in British Columbia and moved with his family to Seattle when he was 3 years old. His father owned MacDonald Meat Co., and profits from that business formed the basis of the MacDonald trust.

MacDonald married Mary Katherine Moore, a widow with two grown children, and moved into her modest Magnolia home in 1971, said Dennis, the stepdaughter.

“My mother was very vivacious, and she really opened the doors to his life,” Dennis said. She convinced him to travel, and the two took extensive tours throughout Europe and traveled to Australia, Canada and Africa.

They moved to Horizon House in 1997, and Mary MacDonald died in 1999.

Dennis said she helped MacDonald write his obituary three years ago after he had a health scare, and he told her that he wanted to be remembered as a philanthropist.

“He felt really good about what he was doing with his money,” Dennis said, “and our family feels good about what he’s doing with his money.”

Katherine Long: 206-464-2219 or:

klong@seattletimes.com

Twitter: @katherinelong