No, there are many ways to work in architecture that do not include the Bachelor of Architecture (BArch). In most states (not all) you need to have either a BArch or a Master of Architecture in order to become a licensed architect. With a BA or BS in architecture, art studio, art history OR ANYTHING ELSE you can get into a good MArch program as long as you fulfill the program’s requirements.
The requirements vary from school to school but generally include some art studio, some art history, some calculus and some physics. The portfolio, which may or may not include architecture-driven artwork, is a major part of the MArch admissions process.
The MArch will take from 2.0 to 3.5 years depending on the MArch school and your undergraduate degree. It’s quite common to work in architecture for a couple of years before heading back to graduate school.
It is also common to work as an architect (often called an architectural designer) without being a licensed architect; however, getting a BArch or MArch (or both) and going through the licensing process is the recommended route.
As I said, the undergraduate degrees of students accepted into MArch programs vary widely; however, your choice of degree may impact the time that it takes to complete the MArch and will therefore impact the total cost. MArch programs do give grants and scholarships but funding is limited, so you have to think carefully about the cost of an additional 2.0 to 3.5 years of education. Entry level architecture jobs are underpaid relative to the education commitment required.
I don’t know much about Harvard’s track record for getting graduates into good MArch programs (including their own!) but I’m sure this is a question that would be easy to find the answer to. Whether you major in art, art history or whatever interests you, you just have to make sure that you fulfill the MArch admissions requirements and devote a huge amount of energy to your portfolio.
Universities that have MArch programs are often good choices for undergraduate architecture, whether they offer the BA, BS or Bachelor of Environmental Design. Check the curriculum, though, as some undergraduate architecture or architectural studies programs – like the one at MIT – can be highly intensive and may not allow much room for double majoring. You could look at WUSTL, Tulane, Virginia, Northeastern, Cinncinnati, UMich, Colorado Boulder. Although Brown doesn’t have a school of architecture, they do have an excellent undergraduate architecture program.
Among small liberal arts colleges, Conn College is a good choice as are Middlebury, Williams, Hamilton, Vassar. I’m not familiar with Colgate, though the pre-architecture program in the link provided by @markham looks interesting. I would be less enthusiastic about Amherst.
Study the curriculums carefully, as there’s a lot of variation from school to school. At some you will enter directly into the undergraduate architecture program (and may have to submit a portfolio). At some you will not declare you major until sometime in your sophomore year.
Ask which MArch programs their graduates have gone on to. Most importantly don’t underestimate the cost of the BA/BS+MArch route.
Lastly many schools of architecture hold open houses in the fall to provide information about what they are looking for in admissions. Yale’s is on November 2, Harvard on November 3. Cornell on November 6. Try to attend one. I promise you it will be an eye-opener.