US Marriage Legal in the UK

Does a marriage that took place in the USA between an American and a Brit have to be registered with a government entity in the UK in order for the marriage to be legal/recognized? 

No, I don't believe so.

A marriage that is valid and legal in the US, is also automatically considered valid and legal in the UK.

Thank you.  A US attorney told me that I would be required to register in the UK, but I could find no info supporting his statement.

Thank you.  A US attorney told me that I would be required to register in the UK, but I could find no info supporting his statement.

Nope, not required. As ksand said, if it's legal in the US, it's legal in the UK.

From the UK government website:

Getting married or forming a civil partnership abroad

Find out about who to contact and which documents you may need to get from the UK authorities if you want to get married or form a civil partnership abroad.

Your overseas marriage or civil partnership will be recognised in the UK if you follow the correct process according to local law - you won’t have to register it in the UK.

(https://www.gov.uk/marriages-civil-partnerships/what-you-need-to-do)

2. Marriage overseas

2.1 For a foreign marriage to be valid in English law, the following conditions must normally be fulfilled:

• The marriage ceremony must be recognised as a valid form of marriage by the law of the place of celebration (mixed marriages and "common law" marriages are sometimes excepted - see paragraphs 9-10 below)

•  Each of the parties to the marriage must have the capacity, under the law of the place where he or she is domiciled at the time of the marriage, to marry the other party in the manner proposed (see DOMICILE)

•  Any previous marriage of either party must first have been validly terminated in the eyes of English law (but see paragraph 8 below)

(More on this here: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/268020/marriage.pdf)

As the others have said, if you are married in the US, you are married in the UK.  No need to do anything else.  You also do not need an Apostille (a poor attorney may recommend this as well).