Severe combined immunodeficiency (SCID) is fatal unless treated with hematopoietic stem cell transplant. Delay in diagnosis is common without newborn screening. Family history of infant death due to infection or known SCID (FH) has been associated with earlier diagnosis. The aim of this study was to identify the clinical features that affect age at diagnosis (AD) and time to the diagnosis of SCID. From 2005 to 2016, 147 SCID patients were referred to the Asian Primary Immunodeficiency Network. Patients with genetic diagnosis, age at presentation (AP), and AD were selected for study. A total of 88 different SCID gene mutations were identified in 94 patients, including 49 mutations, 12 mutations, 8 mutations, 7 mutations, 4 mutations, 4 mutations, 2 mutations, and 2 mutations. A total of 29 mutations were previously unreported. Eighty-three of the 94 patients fulfilled the selection criteria. Their median AD was 4 months, and the time to diagnosis was 2 months. The commonest SCID was X-linked (  = 57). A total of 29 patients had a positive FH. Candidiasis (  = 27) and bacillus Calmette-Guérin (BCG) vaccine infection (  = 19) were the commonest infections. The median age for candidiasis and BCG infection documented were 3 months and 4 months, respectively. The median absolute lymphocyte count (ALC) was 1.05 × 10 /L with over 88% patients below 3 × 10 /L. Positive FH was associated with earlier AP by 1 month (  = 0.002) and diagnosis by 2 months (  = 0.008), but not shorter time to diagnosis (  = 0.494). Candidiasis was associated with later AD by 2 months (  = 0.008) and longer time to diagnosis by 0.55 months (  = 0.003). BCG infections were not associated with age or time to diagnosis. FH was useful to aid earlier diagnosis but was overlooked by clinicians and not by parents. Similarly, typical clinical features of SCID were not recognized by clinicians to shorten the time to diagnosis. We suggest that lymphocyte subset should be performed for any infant with one or more of the following four clinical features: FH, candidiasis, BCG infections, and ALC below 3 × 10 /L.