Short answer: Subway uses a commercially produced mayonnaise for its sandwiches, but the exact brand isn’t officially published by Subway. Details and context:

• Subway’s own site lists several sauces and mayo-based options (including regular mayonnaise) as signature condiments, but it does not publicly disclose the specific supplier or brand for its standard mayonnaise [subway.com]. This means the exact formulation or supplier is kept internal by Subway.
• In practice, many fans and reviewers speculate about brands like Hellmann’s, Duke’s, or other large-scale egg-based mayonnaises, often noting that Subway’s mayo tastes tangier or more vinegary than typical store brands. However, these are anecdotal and not official confirmations from Subway [reddit discussions, third-party sources].
• If you’re aiming to replicate Subway mayo at home, there are popular copycat recipes that attempt to mimic its flavor by adjusting oil blends, vinegar content, salt, lemon juice, and mustard—though these are unofficial approximations and vary by recipe [copycat sites and blogs].

What this means for you:

• If you need to know the exact Mayo supplier for Subway for sourcing or compliance reasons, there isn’t a publicly available official confirmation. You’d likely need to contact Subway corporate or a franchise-level procurement representative for a definitive answer.
• For practical purposes (e.g., home cooking or menu planning), you can approximate Subway mayo using a standard full-fat mayonnaise base with a touch more vinegar/lemon juice and a pinch of mustard to mimic the tang and creaminess.

If you’d like, I can:

• Locate recent official statements from Subway about sauces or condiments for precise wording.
• Compare common mayonnaise brands’ ingredients to identify which ones most closely resemble Subway-style mayo.
• Provide a tested copycat recipe that replicates Subway mayo’s tanginess and creaminess.