The tone was bittersweet Monday morning at One World Trade Center, where office employees for the first time arrived for work at the skyscraper that stands near where the Twin Towers fell in 2001.

“Considering what went on here 13 years ago, it was very tragic, but we are going forward and I think it’s wonderful to have this opportunity to work in the biggest building in the western hemisphere,” said Mary Ann Casey, a paralegal heading into the new building.

Ms. Casey is among 175 people who work for Condé Nast or its corporate affiliates and who have moved into the 104-story tower. Ultimately, some 3,400 employees of the media company will move into the new offices over the next several months.

Another worker marking her first day at the building, executive assistant Wendy Schoeler, said she was “nervous, excited and feeling hopeful and positive about it. I am thinking it’s a new chapter and a new start.”

The $3.9 billion, 1,776-foot skyscraper overlooks the National September 11 Memorial Museum and the memorial plaza’s reflecting pools in the footprint of the Twin Towers.