Decide on the type of agency
Before moving to an agency, research it well. Decide whether to work in a small, large, or multinational company. The benefits from the large agency may be that you can work in a different team more easily. For example, if you work in affiliate marketing or PPC and transition into SEO, you may be able to keep working on the same brand but switch verticals.
It can be hard to decide which agency to work with. Here is a list of some SEO agencies in the UK and those which are in the US. There are also agencies that allow you to do remote work, but they are harder to find. The career path in large SEO agencies tends to be more organized and is along the lines below:
SEO assistant or SEO intern > SEO Executive > SEO Manager > Senior SEO Manager > SEO Director > Head of SEO
Benefits of working at an agency
Those at agencies have the opportunity to work on several accounts and gain experience across a range of different industries. This is good for those who are starting out either straight from school or college or those who have transitioned. It tends to be fast-paced and full of activity.
Agency work tends to be longer hours. This is particularly true when working on pitches, as this is often done outside of office hours. However, this can depend on the agency; not all agencies expect their staff to work beyond normal working hours. Working on pitches builds up your own experience, and you also have exposure across different teams; as for some pitches, you may collaborate with paid search, affiliates, or other marketing departments.
Working at agencies can positively help your career as you can learn a great deal from people around you. There are many creative people who you would not usually meet in-house.
Another advantage of working at an agency, particularly a large one, is having an experienced senior team. This gives you the opportunity to learn from your senior leaders and also from your peers. This is something that might not be possible in a client-side role.
A crucial part of carving a career in SEO is learning, and I have found a major source of learning and practical tips can be picked up at conferences and events. At agencies, you are exposed to more of these conferences and may have more of an opportunity to attend them, especially if your agency is one of the organizers.