Do you need an LLC for a marketing agency?
Legally, no. You can operate as a sole proprietor in most US states. Practically, yes. Most professional US marketing agencies operate as LLCs (or S-corps at larger scale) to separate personal and business liability, simplify client contracting, and enable tax elections that reduce self-employment burden. Total setup is typically $100–$500 depending on state; the bigger investment is a competent CPA for the first year.
This question turns on judgment rather than on vendor data. The Wall's index holds no figure that bears on it, so none is attached. The answer above is editorial.
Why this matters
Google puts this question in People Also Ask for the "marketing agency" query, which means buyers are asking it while they shop. The answer above is The Wall's editorial position. We index 3,460 verified US growth-services vendors across 10 pillars, and this particular question sits outside what that data can settle.
If you are evaluating vendors in this category, the Marketing pillar page carries the full US firm list with rate, team size and headquarters filters.