PORTLAND – In a big career that includes the John Hancock Tower in Boston, Collins Place in Melbourne, Australia, and the World Trade Center in Baltimore, as well as numerous prestigious teaching posts, architect Henry N. Cobb has a special fondness for a small but significant project, the Charles Shipman Payson Building at the Portland Museum of Art, for which he was chief designer.

The Portland project, Cobb said, remains one of the most memorable and magnetic of his career, and will, in part, be the theme of a keynote address he will deliver 6 p.m. Thursday, May 1, at the museum’s kickoff of a monthlong 20th anniversary celebration of the Payson wing. Cobb, who is a founding member of the New York architectural firm headlined by I.M. Pei, will also deliver a formal lecture titled “The Skyscraper as Citizen: Reflections on the Public Life of Private Buildings.”

Jewel-like though the Payson may be in the scope of Cobb’s shining contributions to cityscapes, it was no small controversy in 1983 when it opened to the public. The brick building, with its soaring facade of see-through circles and street-level arcade, dominated Congress Square – and still does – and critics found it entirely too modern.

These days, the Payson is a beloved beacon for a thriving arts district.

“What Henry Cobb was doing was creating a very dramatic new entrance and new image for the Portland Museum of Art,” said Earle G. Shettleworth Jr., director of the Maine Historic Preservation Commission in Augusta. “As you look back 20 years, the shock of the new has mellowed and, if anything, we now recognize the genius of what he was doing in being very sensitive to the material and vocabulary of the city of Portland and its traditions.”

Cobb, who lives in Manhattan and has a home on North Haven, began studying the traditions of Portland in the 1960s, when the museum invited him to participate in a series of lectures on architecture. He arrived early in town to examine more thoughtfully a city he had been driving by every summer since the 1930s, when his family purchased a farmhouse on North Haven. He knew of ancestors who had lived and worked in Portland – in a strange twist of kismet, his great-great-great-grandfather had a house on the very site of the Payson building – and he was expecting to uncover more about them. But Cobb was not prepared for his own passionate response to the winding thoroughfares, vistas and waterfront in Portland.

“I was totally swept off my feet,” said Cobb, who grew up in Boston and studied architecture at Harvard with the legendary Bauhaus leader Walter Gropius. “I fell in love, and a historical analysis of Portland and its growth became the subject of my lecture.”

The city also lodged itself indelibly in his professional thoughts. At the time, the federal government’s urban renewal programs were sweeping the country, and Portland had extensive plans intended to bring business to the downtown and stem the flow of resources into the suburbs.

Historic landmark and preservation commissions were not yet established in the region. Consequently, the plan for growth, in the eyes of city planners, was sure to involve initial destruction of streets and buildings. They saw their project as hopeful; Cobb perceived it as an evisceration of one of the most beautiful systems of public ways he knew. He took on Portland as a personal project and pitched a plan of his own. Sadly for Cobb, it was too late.

By that time, Cobb had forged a relationship with the members of the museum, and when the plan to build the new wing developed, Cobb’s was one of four world-class architectural firms to submit designs. In the official report that awarded the job to I.M. Pei & Partners, the museum committee cited, among other qualities, the “extraordinary talents and commitments of Henry Cobb” as instrumental in their final choice.

“We saw it as a unique opportunity for what we thought could well be the most important architectural moment for the state of Maine,” said Leonard Nelson, an attorney in Portland and chairman of the architect selection committee in the 1970s. “My personal goal was that there be two, maybe three mandatory stops in Maine: L.L. Bean, Acadia National Park, and the Portland Museum of Art. That has come to pass. The museum helped transform the city at the same time that a lot of other things were happening, too. It was a bold vision, and Maine isn’t loaded with bold visions.”

The Payson Building, which cost $11.6 million to build, connects through hallways to two existing historical landmarks in the museum complex: the McLellan House, built in 1801 and now an example of Federal architecture and style, and the L.D.M. Sweat Memorial Galleries, a Beaux Arts museum that contains a permanent collection of 19th century American art. In Cobb’s overall plan, the museumgoer could essentially enter the museum in 1800s architecture and end up in the 20th century, or enter from another direction and take a tour the other way round.

That design dream was unrealized until last year, when the two older buildings were restored and reopened after 20 years. Cobb will see the completion of his dream for the first time this week.

“For me, it was extremely important that the Payson building was not a freestanding, autonomous structure,” said Cobb. “It was an addition meant to operate with earlier galleries. It was unique that architectural history could be experienced in walking through the galleries. You could see the evolution of tastes and styles. It’s an example of one and one and one adding up to more than three. I am proudest of all that the Payson building is a good neighbor to the other two buildings. The museum is finally whole.”

20 Days in May: The Charles Shipman Payson Building 20th Anniversary at the Portland Museum of Art

Thursday, May 1: Kickoff celebration with architect Henry N. Cobb, 6 p.m. at the museum.

Friday, May 2: Bring your family and play gallery game of 20 questions.

Saturday, May 3: The first 20 museum cafe customers will receive a free slice of birthday cake.

Sunday, May 4: The 20th visitor that day will win a private tour for four people of the museum with director Daniel E. O’Leary.

Tuesday, May 6: The 20th visitor wins an individual membership.

Wednesday, May 7: Enter a drawing to win a $20 gift certificate to the museum shop.

Thursday, May 8: Receive free admission if your birthday is on the 20th of any month.

Friday, May 9: Spend $20 in the museum shop and receive a free T-shirt.

Saturday, May 10: Enter a drawing to win $20 gift certificate to the museum cafe.

Sunday, May 11: Mothers get in for 20 cents all day.

Tuesday, May 13: Special tours of art from the 1920s at 10:20 a.m., 12:20 and 2:20 p.m.

Wednesday, May 14: Free admission if you are 20 years old.

Thursday, May 15: Free admission to couples married at least 20 years.

Friday, May 16: Purchase two choice posters in the Museum Shop for $20.

Saturday, May 17: Museum tours will take place at 20 past the hour beginning at 10:20 a.m.

Sunday, May 18: $20 group rate for 10 people maximum (with two adults present).

Tuesday, May 20: Eat free in the museum cafe if that day is your birthday.

For information about the museum or events, call 775-6148.