just had this situation when i helped a friend request mileage by creating a mileage plus account well after the flight was flown. here's the customer service response:


MileagePlus members may receive mileage credit for flights operated by
United, United Express, Copa, and Copa Colombia taken up to 30 days
prior to enrollment date free of charge.

For United and United Express-operated flights, you may request this
mileage by completing the Mileage Credit Request form using the
following link: www.united.com/retrocredit

For Copa and Copa Colombia-operated flights, please email a copy of all
ticket receipts and boarding passes to [email protected] or mail
the documents to the address below.

In addition, you may receive mileage credit for flight activity on
United, United Express, Copa, and Copa Colombia between 31 days and six
months prior to enrollment in the MileagePlus program for a $25 USD
service fee.

For United-operated flights, please contact the MileagePlus Service
Center to process your request.*
For Copa and Copa Colombia-operated flights, please mail a copy of all
ticket receipts and boarding passes to:
MileagePlus Service Center
PO Box 6120
Rapid City, SD 57709-6120

If the flights were more than 30 days prior to enrollment, please be
sure to include the MileagePlus account number and a telephone number
along with a credit card number and expiration date. Payment may also be
made by check or money order. Please make the check payable to United
Airlines and include the information from one form of identification
(Driver's License Number, Passport Number or Military I.D. Number). This
information should be placed on the top left hand corner of the check.
As a reminder, all missing mileage requests must be submitted within 12
months of flight date. In addition, other partner flights taken prior to
enrollment in the MileagePlus program are not eligible for mileage
accrual due to contractual agreements with our partners.

*The MileagePlus Service Center is open 7 a.m. - midnight (CT), 7 days a
week. If calling from the U.S. or Canada, please call 800-421-4655. To
reach us from outside the U.S. and Canada, please call 605-348-0011 or
contact your local United reservations office.