Elections to Castlereagh Borough Council were held on 30 May 1973 on the same day as the other Northern Irish local government elections. The election used three district electoral areas to elect a total of 19 councillors.
|- class="unsortable" align="centre" !rowspan=2 align="left"|Ward ! % <br /> !Cllrs<br /> ! % <br /> !Cllrs<br /> ! %<br /> !Cllrs<br /> !rowspan=2|Total<br />Cllrs<br /> |- class="unsortable" align="center" !colspan=2 bgcolor="" | <span style="color:black;">UUP</span> !colspan=2 bgcolor="" | <span style="color:black;">Alliance</span> !colspan=2 bgcolor="white"| <span style="black;">Others</span> |- |align="left"|Area A |bgcolor="#40BFF5"|55.4 |bgcolor="#40BFF5"|3 |25.8 |2 |18.8 |1 |6 |- |align="left"|Area B |bgcolor="#40BFF5"|41.2 |bgcolor="#40BFF5"|4 |20.8 |2 |38.0 |2 |8 |- |align="left"|Area C |bgcolor="#40BFF5"|59.6 |bgcolor="#40BFF5"|3 |19.9 |2 |20.5 |1 |5 |- class="unsortable" class="sortbottom" style="background:#C9C9C9" |align="left"| Total |50.6 |10 |22.1 |5 |27.3 |4 |19 |-
1973: 3 x UUP, 2 x Alliance, 1 x United Loyalist
1973: 4 x UUP, 2 x Alliance, 1 x United Loyalist, 1 x Loyalist Coalition
1973: 3 x UUP, 1 x Alliance, 1 x Independent