#Excel, 63 bytes

=A1&MID("thstndrdth",MIN(9,2*RIGHT(A1)*(MOD(A1-11,100)>2)+1),2)

(MOD(A1-11,100)>2) returns FALSE when A1 ends with 11-13

2*RIGHT(A1)*(MOD(A1-11,100)>2)+1 returns 1 if it's in 11-13 and 3,5,7,etc. otherwise

MIN(9,~) changes any returns above 9 into 9 to pull the th from the string

MID("thstndrdth",MIN(~),2) pulls out the first th for inputs ending in 11-13, st for 1, nd for 2, rd for 3, and the last th for anything higher.

=A1&MID(~) prepends the original number to the ordinal.

_{Posting as wiki since I am not the author of this. (Source)}

Excel, 63 bytes

=A1&MID("thstndrdth",MIN(9,2*RIGHT(A1)*(MOD(A1-11,100)>2)+1),2)

(MOD(A1-11,100)>2) returns FALSE when A1 ends with 11-13

2*RIGHT(A1)*(MOD(A1-11,100)>2)+1 returns 1 if it's in 11-13 and 3,5,7,etc. otherwise

MIN(9,~) changes any returns above 9 into 9 to pull the th from the string

MID("thstndrdth",MIN(~),2) pulls out the first th for inputs ending in 11-13, st for 1, nd for 2, rd for 3, and the last th for anything higher.

=A1&MID(~) prepends the original number to the ordinal.

_{Posting as wiki since I am not the author of this. (Source)}

#Excel, 63 bytes

=A1&MID("thstndrdth",MIN(9,2*RIGHT(A1)*(MOD(A1-11,100)>2)+1),2)

#Excel, 63 bytes

=A1&MID("thstndrdth",MIN(9,2*RIGHT(A1)*(MOD(A1-11,100)>2)+1),2)

(MOD(A1-11,100)>2) returns FALSE when A1 ends with 11-13

2*RIGHT(A1)*(MOD(A1-11,100)>2)+1 returns 1 if it's in 11-13 and 3,5,7,etc. otherwise

MIN(9,~) changes any returns above 9 into 9 to pull the th from the string

MID("thstndrdth",MIN(~),2) pulls out the first th for inputs ending in 11-13, st for 1, nd for 2, rd for 3, and the last th for anything higher.

=A1&MID(~) prepends the original number to the ordinal.

_{Posting as wiki since I am not the author of this. (Source)}